The present invention relates in general to amplitude modulation circuits and in particular to use of feedback to stabilize modulation frequency response of an amplitude modulation circuit.
The carrier output of a radio frequency (RF) signal generator can be modulated with an audio frequency signal, thereby to produce an amplitude modulated RF output signal. A circuit for effecting this result advantageously employs a leveling loop including a feedback circuit generating a modulating signal proportional to the difference between amplitudes of the audio frequency input and the detected envelope of the RF output signal. In such case, the amplitude of the modulating signal controls the amplitude of the RF output signal, with the leveling loop stabilizing the RF output so that the output envelope tracks the audio input. However, the response of the circuitry including the leveling loop can change with the RF carrier frequency, particularly at higher modulation frequencies. Since users normally desire the RF output signal response to remain constant despite changes in carrier frequency, the above interdependence is undesirable, particularly with respect to test instruments and signal generators.